The accurate detection and measurement of radiation is employed in many industries including homeland security, scientific instrumentation, medical imaging and the minerals processing industry. These and other industries use the detection and measurement of radiation for the non-invasive analysis of materials or other specimens. Transmission based imaging, spectroscopic analysis or other modalities can be used to perform such analysis.
Spectroscopy, for example, is commonly used to analyze materials. Knowledge about the material is obtained by analysis of radiation emission from elements within the specimen. This emission of radiation can be stimulated emission due to some form of incident radiation or the result of natural emission from the constituent elements.
Gamma-ray spectroscopy, for example, is a form of spectroscopy in which the emitted electromagnetic radiation is in the form of gamma-rays. In gamma-ray spectroscopy the detection of the resulting radiation is commonly performed with a scintillation crystal (such as thallium-activated sodium iodide, NaI(Tl)), though there are a number of other detector types that can also be used. NaI(Tl) crystals generate ultra-violet photons pursuant to incident gamma-ray radiation. These photons may then be directed to a photomultiplier tube (PMT) which generates a corresponding electrical signal or pulse. As a result, the interaction between the photons and the detector gives rise to pulse-like signals, the shape of which is determined by the incident gamma-ray radiation, the detecting crystal and the PMT. The fundamental form of these pulse-like signals is referred to as the impulse response of the detector.
The output from the photomultiplier is an electrical signal representing the summation of input signals, of determined form, generated in response to discrete gamma rays arriving at the scintillation crystal. By examining the detector output over time, and in particular the amplitude of the component signals, it is possible to deduce information regarding the chemical composition of the material.
Analysis by gamma-ray spectroscopy requires the characterization of the individual signals generated in response to incident gamma-rays. Signal parameters of particular interest include signal amplitude, number and time of occurrence or temporal position (whether measured as time of arrival, time of maximum or otherwise). If the arrival times of two gamma-rays differ by more than the response time of the detector, analysis of the detector output is relatively straightforward. However, in many applications a high flux of gamma-rays cannot be avoided, or may be desirable so that spectroscopic analysis can be performed in a reasonable time period. As the time between the arrivals of gamma-rays decreases, characterization of all resultant signals becomes difficult.
In particular, the analysis is affected by a phenomenon known as pulse pile-up [G. F. Knoll, Radiation Detection and Measurement, 3rd edition, Chapter 17, pp. 632-634, 658 and 659, John Wiley and Sons, New York 2000], whereby multiple gamma-rays arriving more or less simultaneously produce signals which sum together and may be counted as a single signal. The magnitude of this combined signal is greater than the individual components, leading to errors in later analysis.
The energy of an incident gamma-ray may be reflected in the amplitude of the pulse-like signal produced by the detector. The presence of specific gamma-ray energies within the detector signal is indicative of particular elements in the material from which gamma-rays originate. Thus, a failure to differentiate a large amplitude signal caused by a single scintillation event from the superposition of multiple events can have a serious effect on the accuracy of subsequent spectroscopic analysis.